A typical gas turbine includes an inlet section, a compressor section, a combustion section, a turbine section, and an exhaust section. The inlet section cleans and conditions a working fluid (e.g., air) and supplies the working fluid to the compressor section. The compressor section progressively increases the pressure of the working fluid and supplies a compressed working fluid to the combustion section. The compressed working fluid and a fuel are mixed within the combustion section and burned in a combustion chamber to generate combustion gases having a high temperature and pressure. The combustion gases are routed along a hot gas path into the turbine section where they expand to produce work. For example, expansion of the combustion gases in the turbine section may rotate a shaft connected to a generator to produce electricity.
The combustion section generally includes one or more combustors annularly arranged and disposed between the compressor section and the turbine section. Various parameters influence the design and operation of the combustors. For example, gas turbine manufacturers are regularly tasked to increase gas turbine efficiency without producing undesirable air polluting emissions. The primary air polluting emissions typically produced by gas turbines burning conventional hydrocarbon fuels are oxides of nitrogen (NOx), carbon monoxide (CO), and unburned hydrocarbons (UHCs). Oxidation of molecular nitrogen and thus the formation of NOx in air-breathing engines such as gas turbines is an exponential function of temperature. The higher the temperature of the combustion gases, the higher the rate of formation of the undesirable NOx emissions.
One way to lower the temperature of the combustion gases, thus controlling the formation of NOx, is to pre-mix fuel and air upstream from a combustion reaction zone within the combustion chamber using a premix type of fuel injector or fuel nozzle assembly, such as a swirler or swozzle type fuel nozzle assembly. In this type of fuel nozzle assembly, fuel is injected into a flow of compressed air within an annular flow or premix passage defined within the fuel nozzle assembly. The fuel and compressed air mixes within the annular passage and is then routed into the combustion chamber from a downstream end of the fuel nozzle assembly. During combustion, the heat capacity or thermal capacitance of excess air present in the air-rich or fuel-lean combustible mixture absorbs heat in the combustion chamber, thus reducing the temperature of the combustion gases, thereby decreasing or preventing the formation of NOx emissions.
A flow field of the lean combustible mixture within the premix passage and entering the combustion chamber at the injection point should be uniform or symmetric in order to reduce the potential for flame holding and to achieve desired emissions performance. Accordingly, continued improvements in current fuel nozzle assembly technologies would be useful.